In recent years, influences of particulate matters or NOx exhausted from an engine for an automobile, especially a diesel engine or the like onto environments have been brought into the public eye, and various uses of porous honeycomb structures as important means for removing these toxic substances have been studied.
For example, in a honeycomb structure including a plurality of through-holes partitioned by porous partition walls, a honeycomb filter has been developed including a structure in which the through-holes are plugged in different positions on opposite end faces including the through-holes opened therein. An exhaust gas is allowed to flow in each through-hole opened in one end face, and forcibly passed through the partition walls in the honeycomb structure to trap and remove the particulate matters in the exhaust gas. As new attempts to increase a catalyst support amount and to thereby improve a purifying performance, a catalyst body of a honeycomb structure has also been developed including the partition walls which are all formed into porous bodies having high porosities and which support a catalyst for decomposing HC or NOx.
Additionally, in the porous honeycomb structure, a high trapping efficiency has been naturally required in the application as the filter. When a certain or more amount of soot is deposited on pores opened in the surfaces of the partition walls, the pressure loss rapidly increases. Therefore, usually after the use for a certain time, a regeneration step is performed to burn the filter at a high temperature and to thereby burn up the soot. However, when this regeneration step is performed with a high frequency, degradation of the porous honeycomb structure is promoted. Therefore, a trapping time has been required to be lengthened to decrease the number of regeneration steps.
Furthermore, in the application as the filter, since the porous honeycomb structure is exposed at a remarkably high temperature at the time of the filter regeneration, the whole filter preferably has a certain or more thermal capacity in order to prevent a dissolved loss of the partition wall by the filter regeneration. When the soot is burnt, a maximum soot amount (soot limited regeneration amount) that does not cause the dissolved loss of the partition wall during the regeneration of the filter is required to be large.
On the other hand, in the application as the catalyst carrier, in recent years, there has been a demand for the increasing of a catalyst support amount for further improvement of an exhaust gas purifying performance, and attempts have been made to support the catalyst on the honeycomb structure whose porosity has been raised.
Moreover, in any application, there has been a demand for the reduction of the pressure loss in the porous honeycomb structure installed in a combustion engine such as a diesel engine strongly required to have reduced fuel consumption and increased output. Furthermore, since the porous honeycomb structure is disposed in the vicinity of the engine continued to be largely vibrated in any application, the structure needs to be firmly grasped in a metal case, and the whole honeycomb structure is required to have a high isostatic strength.
To meet these demands, a honeycomb structure or the like has heretofore been disclosed in which the pore distribution of the partition walls is controlled in various ranges.
For example, in Japanese Patent No. 2726616, a honeycomb structure has been described in which a specific surface area (Mm2/g) of the pore opened in the surface of the partition wall and a surface roughness (Nμm) in the filter surface are controlled in 1000M+85N≧530.
However, the honeycomb structure is manufactured using ceramic raw materials such as silica and talc whose particle diameters have been controlled, and the porosity is about 60% at maximum (Japanese Patent No. 2726616).
Moreover, in the honeycomb structure, the distribution of the pores inside the partition walls, except a ratio of the pores opened in the surface of the partition wall with respect to the porosity of the whole partition wall, is not considered, and demands for the lengthening of the trapping time, the raising of the trapping efficiency, the improving of the purifying performance, and the lowering of the pressure loss have not been sufficiently satisfied.
On the other hand, a cordierite honeycomb structure (Japanese Patent Application Laid-Open No. 9-77573) has been proposed in which an organic foaming agent and carbon are added as pore formers to a cordierite raw material for use, and the number of 5 to 40 μm small holes is set to be five to 40 times that of 40 to 100 μm large holes among the pores in the surface of the partition wall.
However, also in this honeycomb structure, the ratio of the pores opened in the partition wall surface with respect to the porosity of the whole partition wall has not been considered. The honeycomb structure has been manufactured using an organic foaming agent which originally contains dense particles and which is hollowed when heated as a pore former material. Therefore, there is little organic foaming agent resulting in the opened surface of the partition wall just after extrusion molding. When the binder gels by heat in the subsequent drying step or the like, and the formed article is hardened, the surface of the partition wall is not largely expanded in such a manner as to be burst even by the foaming at a comparatively low temperature of 100° C. or less, and the number of pores opened in the partition wall surface has been small in the present situation. As a result, in the honeycomb structure, the pore distribution of the partition wall in a thickness direction has a deviation, and the demands for the raising of the trapping efficiency, the lengthening of the trapping time, and the lowering of the pressure loss have not been sufficiently satisfied. Since the amount of the catalyst supported inside the partition walls is very large because of the deviation of the pore distribution, an effective use ratio actually contributing to a purifying reaction is small, and a sufficient purifying performance has not been obtained. Furthermore, when the porosity of the whole partition wall is further raised in order to solve the problem, there has been a problem that a local dissolved loss on the partition wall at the time of the filter regeneration is caused by the decrease of the isostatic strength or the thermal capacity.
Moreover, even in the honeycomb structure manufactured using non-foaming pore formers such as PMMA and PET, the number of pores opened in the partition wall surfaces has been small in the present situation as described above. The demands for the effective raising of the trapping efficiency, the lengthening of the trapping time, the improving of the purifying performance, and the lowering of the pressure loss are not sufficiently satisfied. When the porosity of the whole partition wall is further raised, there has been a problem that the local dissolved loss of the partition wall at the time of the filter regeneration is caused by the decrease of the isostatic strength or the thermal capacity.